Pervaporation is a low-pressure membrane process that can be used to separate components of differing volatilities from solutions. The membranes used are selectively permeable (permselective) to one component of the feed solution. Transport through the membrane is induced by maintaining the vapor pressure on the permeate side of the membrane lower than the vapor pressure of the feed mixture. The driving force for pervaporation is the difference in partial vapor pressure of each species across the membrane. One or more of the feed liquid components pass through the membrane in vapor form. The non-permeating fraction is removed as a liquid residue.
Gas separation is also a pressure-driven membrane process. In this case, a feed gas mixture at a certain pressure contacts one side of a permselective membrane and a lower pressure is maintained on the permeate side. The components of the mixture diffuse through the membrane under a potential gradient brought about the pressure drop across the membrane. Vapor separation is a type of gas separation application in which the feed gas contains organic vapors that are to be removed, typically, from air.
The optimum permselective membrane for use in any of these applications combines high selectivity with high flux. These two basic properties are determined by the membrane materials and the membrane thickness. As a general rule, high flux and high selectivity are mutually contradictory properties. Polymers with high selectivities for one component over another tend to be relatively impermeable; highly permeable materials on the other hand tend to be unselective. Thus the membrane-making industry has engaged in an ongoing quest for membranes with improved flux/selectivity performance.
One way to utilize highly selective materials and reduce the effect of low permeability is to make the membrane extremely thin. One way to minimize membrane thickness is to prepare a composite membrane consisting of a thin film coated onto a microporous support. Such a membrane is characterized on the basis of whether the coating or the support controls the separation properties. If the microporous support has a very high surface porosity, gas transport through the support will take place by convective flow and/or Knudsen diffusion through the pores. On the other hand, if the microporous support has a surface porosity less than about 10.sup.-4, most of the gas transport will take place by diffusion through the polymer phase, so that the support rather than the coating determines the separation properties. This is the case, for example, with the membranes described in U.S. Pat. No. 4,230,463, to Henis and Tripodi, which are now sold commercially under the name Prism.RTM.. Membranes where the coating layer provides the separation properties are described, for example, by Ward et al. in "Ultrathin silicone rubber membranes for gas separation", J. Membrane Sci. 1, 99, 1976, by Riley et al. in "Development of ultrathin membranes", Office of Saline Water Report No. 386, PB# 207036, and in U.S. Pat. No. 4,553, 983 to Baker. In addition, there are numerous other references in the literature describing attempts to make ultrathin, defect-free membranes. Much of this work has been performed in the belief that the selectivity of the composite membrane will remain essentially the intrinsic selectivity of a thick film of the permselective material, but that the transmembrane flux will increase as the permselective layer is made thinner. Therefore, it is customary to ascribe any loss of selectivity observed with ultrathin membranes to defects in the permselective layer, through which unselective bulk transport of materials takes place.